Let Herald activation depend on which transactions are being applied, and generate transactions

Summary:
Ref T2222. Ref T4484. This is a stepping stone to getting Herald supported in the new Differental code. Generally:

• Instead of an Editor either supporting or not supporting Herald, let it choose based on transactions. Specifically, Differential only runs rules on revision creation and diff updates.
• Optionally, allow an Editor to return some transactions to apply instead of having to apply everything itself. This lets us make it clear why changes happend in the transaction log, and share more code.
• I updated only one transaction type (owners in Maniphest) since it was the easiest and cleanest to update and test. Everything else still works like it used to, it just won't generate a transaction record yet.
• The transaction records are a touch rough, but we can clean them up later.

Test Plan: {F122282}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4484, T2222

Differential Revision: https://secure.phabricator.com/D8404